Rotors for turbo-generators are known which comprise an iron core with two pole parts and an intermediate, substantially parallelepipedic portion surrounded by a plurality of winding turns which together form the excitation winding of the generator. Two shaft ends are flanged to one end each of the iron core with separate intermediate circular plates of non-magnetic material and a pre-stressed cylindrical metal casing is arranged to surround the whole iron core and exert a pressure against the air gap surface of said iron core, the pressure being evenly distributed at least in axial direction. A rotor of the above kind is described in German patent specification DBP No. 1,077,774.